Added headlight intensity configuration to Viewer

The default Viewer headlight's intensity can now be set as a param or by calling the Vizi.Viewer.setHeadlightIntensity(intensity) function. The default value of 1 wasn't bright enough for most of my models. It still defaults to 1 though.

If this pull request is merged, the final build files (vizi.js etc) need to be built again per your specs. When I build them, it looks like my version of closure library is adding additional comments to it or something compared to the existing vizi.js. Also I can't build the minified versions - I get an error about an unspecified file not being found (probably missing a dependency for that but not worth investigating at the moment).